This invention relates to a magnetic recording medium and a method for fabricating the magnetic recording medium used in hard disc drives or the like, and more particularly, relates to a carbon protective film containing hydrogen of the magnetic recording medium and a method for fabrication the carbon protective film.
Hard disc drives or the like are frequently employed as an external storage device for a data processing apparatus such as a computer or the like. In hard disc drives, since a contact start stop method (herein after referred to as CSS) is employed in which a read/write head floats above a recording medium of a magnetic disc while the disc is rotating and the head touches the recording medium while the disc is not rotating, the read/write head is brought to contact to slide on a recording medium when the disc starts or stops rotating. When wear resistance and lubricative properties of a protective film coating a surface of the recording medium is insufficient, abrasion of the protective film proceeds as the aforementioned start-stop cycle is repeated, and in the worst case, a magnetic layer is damaged to cause the head to crash into the disc.
FIG. 10 is a partial sectional perspective view showing a structure of the conventional magnetic recording medium used in hard disc drives. In this magnetic recording medium, a non-magnetic base plate 1 comprises a non-magnetic substrate 1a, and a non-magnetic metal layer (hardening layer) 1b laminated on the substrate 1a. A non-magnetic metal under layer 2 is laminated on the base plate 1. A thin film magnetic layer 3 is formed on the underlayer 2 by laminating a ferromagnetic Co-Cr-Ta (cobalt-chromium-tantalum) alloy layer or a ferromagnetic Co-Cr-Pt(cobalt-chromium-platinum) alloy layer. A protective film 4 is formed on the magnetic layer 3. The magnetic disc is, if necessary, formed by coating a lubricative layer 5 which comprises liquid lubricant on the protective film 4. As an example of the base plate 1, a base plate which comprises Ni-P plated layer 1b formed by electroless plating on a non-magnetic Al-Mg alloy substrate 1a is employed. An alumilite base plate, a glass base plate, a ceramic base plate and the like are also employed. The base plate 1 is polished, if necessary, and is provided with a surface properly roughened by texture formation (texture processed surface). The non-magnetic metal underlayer 2 that comprises Cr; magnetic layer 3 made Co-Cr-Ta alloy as an example; and an amorphous carbon protective film 4 are successively deposited on the base plate 1 by the sputtering method under Ar atmosphere on the base plate 1 heated up to 200.degree. C. Fabrication of the metal thin film disc is completed by coating on the protective film 4 the lubricative layer 5 which comprises a liquid lubricant of the fluorocarbon family.
In the metal thin film recording medium fabricated by the sputtering method, an amorphous carbon film is usually employed as the protective film 4 formed on the magnetic layer 3 by sputtering from a carbon target. Additionally, oxides (e.g., zirconia) are sometimes employed in the protective film 4. Carbon is employed in the protective film 4 because the amorphous carbon layer formed by the sputtering method shows relatively strong graphite-like properties, and shows low coefficient of friction, as graphite specifically does, under the atmosphere containing moisture.
This amorphous carbon protective film shows enough wear resistance and excellent CSS resistance when used with the conventional read/write head of Zn-Mn ferrite with a Vickers hardness of about 650. However, the hardness of the amorphous carbon is much smaller than those of hard ceramic head materials like Al.sub.2 O.sub.3.TiC or CaTiO.sub.3 (Vickers hardness of about 2000) which have been employed recently in a thin film head of the hard disc drives or in a slider of the metal-in-gap (MIG) heads. So, the amorphous carbon protective film tends to be worn out when used with those hard sliders and to crash into the sliders in the worst case because of its insufficient wear resistance and poor CSS resistance with respect to those hard sliders. If a hard protective film made of oxides is employed, the oxide protective film may hardly be worn out, but the oxide protective film may be too hard and its coefficient of friction may be too high; the read/write head crashes instantaneously by head touch in an instantaneous high energy state into foreign substances or projections on the disc surface while the read/write head is being floated during its seek operation or CSS operation.
For solving these problems, it has been disclosed to form on the magnetic layer a protective film comprising a diamond-like carbon film which contains more diamond bonds than graphite bonds by growing diamond-like properties in the carbon film. Various proposals have been made so far on the diamond-like carbon film that shows a high hardness inherent of its diamond structure as well as an excellent sliding property inherent to carbon, and improves wear resistance when used with the hard slider made of Al.sub.2 P.sub.3. TiC or CaTiO.sub.3. Japanese Patent laid open S61-126627 discloses a composite film comprising a hard carbon layer and a fluorine containing lubricative layer formed by the sputtering method or by the CVD method under an atmosphere of gas mixture of an inert gas and a hydrocarbon gas. Japanese Patent laid open H02-71422 discloses a carbon film, film properties of which are identified by hydrogen bond in the film and by its Raman spectrum. Japanese Patent laid open H02-299199 discloses a carbon film, film properties of which are identified by a Raman spectrum. Japanese Patent laid open H02-87322 discloses an example of a magnetic recording medium which comprises a hydrogenated carbon film and a lubricant coated on the carbon film. Japanese Patent laid open H01-258220 discloses a diamond-like carbon protective film containing from 2 to 7.times.10.sup.23 atoms/cc of hydrogen, which shows similar hardness with the hard slider and an excellent CSS resistance. Additionally, Japanese Patent laid open H02-282470 discloses a carbon protective film formed by sputtering in a hydrocarbon gas, which shows a similar hardness with the conventional graphite protective film grown by sputtering in an Ar gas, and is identified to show hydrophobic properties on its surface.
Though we reexamined these disclosed protective films, we failed to reproduce any satisfactory sliding property for the hard slider made of Al.sub.2 O.sub.3.TiC or CaTiO.sub.3. In the carbon protective films described above, wear resistance is improved by forming a hard layer which contains high rate of diamond bonds by growing diamond-like properties in the layer. However, when the protective film is too hard, it damages the magnetic head and the abraded particles enhance abrasion of the magnetic head itself and the magnetic disc. On the other hand, when the protective film is too soft, the protective film is worn out by the hard slider as in the case of the conventional amorphous carbon protective film. As has been explained so far, according to the prior art, any protective film has not been realized that shows an excellent sliding property including a low coefficient of friction and high wear resistance, and an optimum CSS resistance.
In relation to an application of a carbon protective film to the sliders made of Al.sub.2 O.sub.3.TiC or CaTiO.sub.3 while maintaining the low coefficient of friction of graphite-like carbon, U.S. patent application No. 08/142,862 (hereinafter referred to as the "Related Application") describes a carbon protective film with enriched hydrogen content, among the properties of which obtained by the sputtering method hard diamond-like properties are grown on one hand and its hardness is lowered to increase toughness by introducing polymer-like bonds on the other hand. It is described in the Related Application that an excellent CSS resistance against the magnetic head made of hard slider material is obtained by a carbon protective film. The Raman spectroscopic analysis conducted on carbon, a main constituent of the carbon protective film, by the excitation by a 514.5 nm argon ion laser beam reveals that the carbon protective film contains polymer-like bonds and diamond bonds with more content than the coexisting graphite bonds.
FIG. 11 is a graph, described in the related Application, showing relationship between a micro-hardness which is a representation of Vickers hardness of a carbon protective film and methane gas content in an Ar main component sputtering gas. As shown, the micro-hardness increases with increasing methane gas content (in correspondence with hydrogen content increase) and reaches its maximum around methane content of 1 in an arbitrary unit. With further increase of the methane gas content, the micro-hardness decreases. FIG. 12 shows Raman spectra, described in the Related Application, in which FIG. 12 (a) shows a Raman spectrum of a carbon protective film grown in a sputtering gas with zero methane content (hereinafter referred to as "case 1"); FIG. 12 (b) a Raman spectrum of a carbon protective film grown in a sputtering gas with 1 methane content in the arbitrary unit (hereinafter referred to as "case 2"); and FIG. 12 (c) a Raman spectrum of a carbon protective film grown in a sputtering gas with 4 methane content in the arbitrary unit (hereinafter referred to as "case 3"). In the Raman spectrum of FIG. 12 (a) for the case 1, a peak of SP3 level representing diamond bonding shows almost the same height with that of SP2 level representing graphite bonding. The Raman spectrum of FIG. 12 (a) indicates that the graphite-like properties predominate in the carbon protective film which contains a less amount of hydrogen. In the Raman spectrum of FIG. 12 (b) for the case 2, a much higher peak appears at the SP3 level than at the SP2 level. The Raman spectrum of FIG. 12 (b) indicates that diamond bonding predominates over the graphite bonding in the carbon protective film of the case 2, and in correspondence with this the microhardness is large in FIG. 11. FIG. 12 (c) shows the Raman spectrum of a carbon protective film grown in a sputtering gas with a higher methane content, in which a much higher peak appears at the SP3 level than at the SP2 level. The Raman spectrum of FIG. 12 (c) also indicates that the carbon protective film of the case 3 contains high concentration of diamond bonding. We know also from FIG. 12 (c) that the protective film of the case 3 contains many polymer-like bondings, since tails, corresponding to background due to luminescence, are high around each peaks. Accordingly, the hardness of the protective film of the case 3 is estimated to be high because the protective film microscopically contains increased diamond bonds, and toughness of the protective film of the case 3 is estimated to be high due to the increased polymer-like bonds irrespective of decrease in hardness on the micro-scale. And the decrease in the micro-hardness is as large as the micro-hardness of the protective film of the case 3 lowers below that of the case 1 in which the sputtering gas consists only of Ar.
FIG. 13, described in the Related Application, shows a result of a repeated cycle test conducted on a magnetic disc provided with a carbon protective film. The test was conducted with an Al.sub.2 O.sub.3.TiC thin film magnetic head under a load of 10 gf to examine whether the carbon protective film may endure from 25000 to 30000 repetition cycles of a CSS mode as is usually required. As FIG. 13(a) indicates, the coefficient of friction abruptly increases in the magnetic disc with the graphite-rich carbon protective film of case 1 and the magnetic disc crashes in less than 20000 repetition cycles. As FIG. 13(b) indicates, though the coefficients of friction gradually increase in the magnetic discs with the diamond-rich carbon protective film of case 2, crash occurs in early cycles because of brittleness of the carbon protective film caused by its excessive hardness. The carbon protective film of case 2 shows an around 20000 cycles of CSS resistance even when crash did not occur in an early stage of the cycle test. In contrast to the carbon protective film of the cases 1 and 2, as FIG. 13(c) indicates, the coefficient of friction of the carbon protective film of case 3 increases slowly and the carbon protective film of case 3 endures more than 40000 cycles of CSS operation mode with neither any crash nor any hint of deterioration in the coefficient of friction.
FIG. 14, described in the Related Application, in which FIG. 14(a) shows a Raman spectrum of a carbon protective film. In FIG. 14(a), the spectrum shows a peak of the SP2 level representing graphite bonding around 1350 cm.sup.-1 of the Raman shift and a peak of the SP3 level representing diamond bonding around 1560 cm.sup.-1 (1562 cm.sup.-1). In FIG. 14(a), a region S, defined as an area under an asymptote to both tails of the main SP3 peak, corresponds to a contribution from luminescence part of the Raman spectrum. Accordingly, a luminescence intensity ratio of a carbon protective film defined by B/A, where A is a nominal peak height obtained by subtracting the luminescence part from the total SP3 peak height B that includes the luminescence part. The background (region S) of the Raman spectrum corresponding to the contribution from the luminescence part indicates polymer-like bonds in the carbon protective film. It has been found that carbon on protective film shows an excellent CSS resistance when its B/A ratio is more than 1.5. FIG. 14(b) is a wave chart showing two peaks separated by resolving the SP2 and SP3 contributions through the Gaussian distribution function after subtracting by linear approximation the background (region S) attributed to the luminescence of the Raman spectrum. In the figure, peak intensity ratio D/G is defined as ratio of the intensity D of the SP3 peak to the intensity G of the SP2 peak. It has been found that a hydrogen doped carbon protective film shows an excellent CSS resistance when its D/G ratio falls within a range between 1.3 and 3.5.
At a boundary between a hard head slider and a recording medium, elastic deformation occurs on the recording medium and the slider moves while scratching the recording medium. The aforementioned flexible carbon protective film containing more than 35 atomic % of hydrogen shows an excellent CSS resistance, since brittle fracture hardly occurs in the hydrogen rich carbon protective film because of its large elastic deformation. However the hydrogen rich carbon protective film tends to be worn out to deteriorate its CSS resistance when the hydrogen rich carbon protective film is employed with a slider with high grindability (depending on its material and shape) or when the thickness of the carbon protective film is reduced for facilitating high density data storage. A proposed mechanism that estimates the problem described above will be explained below though the proposed mechanism may not afford well detailed explanation. The hydrogen rich carbon protective film shows high coefficient of friction, since nominal contact area is large because of its high flexibility. It is necessary to increase shearing resistance of the carbon protective film for suppressing the coefficient of friction. It is estimated that the shearing resistance is high when the protective film is thick and that the shearing resistance is low and the coefficient of friction is high when the protective film is thin. Therefore, it is estimated that the CSS resistance is deteriorated in association with thickness reduction of the hydrogen rich carbon protective film.